Actually not a terrible signing, as long as the role he plays is appropriate. It's a minor league deal, like the one for Cameron so if the Nats decide to go in another, more productive, direction they can let these guys go, or keep one for the bench or whatever. But don't kid yourself about Ankiel, he's not the lefty bat off the bench Davey wants. He doesn't get on base, he doesn't hit for average, and he's living off reputation in terms of being a power hitter. He hasn't hit for good power in years.That had been, in no small part, to being absolutely useless versus lefty pitchers, but last year he didn't even slug righties well.
No, Ankiel would be a defensive replacement primarily, much like Cameron. Which is fine. You can have that as the last guy on the bench, just not as the last guy in the regular batting order.